This invention relates to traveling wave tubes and in particular to emitting sole crossed field amplifiers and means for improving the collector efficiency of such devices.
The improvement of collector efficiency in traveling wave tubes has been a continuing design objective that has never been fully realized by prior art techniques. These collector efficiency problems and typical prior art approaches to solving them are described in detail in U.S. Pat. No. 3,271,618 of G. P. Kooyers entitled Depressed Collectors For Crossed Field Traveling Wave Tubes. The usual approach in linear beam type devices is to use a depressed collector. The techniques applicable to linear beam type devices are not always useful when applied to crossed field type amplifiers, however. The Kooyers patent discloses a means for improving collector efficiency in crossed field type devices that utilizes a split collector each segment of which is operated at a different depressed voltage. The split elements of the collector are also tapered. The present invention comprehends utilizing a depressed collector and tapering the final stage of the crossed field amplifier interaction region as a means for improving collector efficiency. Tapering can be accomplished physically by tapering the sole emitting cathode structure or electrically by increased tapering of the r.f. slow wave circuit or by shaping the magnetic field. Although the final stages of some linear beam tube slow wave structures have been tapered to keep the electron beam in synchronism with the traveling wave the approach has not been used in crossed field devices which have relatively constant drift velocities. The U.S. Pat. No. 4,087,718 of George F. Farney entitled High Gain Crossed Field Amplifier discloses a crossed field device having a tapered slow wave structure. The taper is, however, for the purpose of increasing device amplification and is a decreasing taper.